In a precise time synchronization protocol IEEE (The Institute of Electrical and Electronic Engineers) 1588 standard, a plurality of nodes connected to a network can share time information by exchanging the time information on the network. When the nodes conforming to the IEEE 1588 standard receive time information from adjacent nodes, the nodes correct time information taking into account a propagation delay of a communication path, prevent occurrence of an error of the time information due to the propagation delay, and realize a high-precision synchronization. Therefore, a measurement of the propagation delay on the communication path has to be accurately performed.
On the other hand, to improve failure resistance on the communication path, a redundant system in which a communication path among nodes is configured by a plurality of lines has been devised.
Japanese Patent Application Laid-open No. 2001-119345 discloses an optical communication system in which an OLT (Optical Line Terminal: a station side terminating apparatus) and a star coupler are connected by two redundant optical fibers (Patent Literature 1).
WO 2008/126162 discloses a protection system that switches a line in use from a working optical fiber to a backup optical fiber when an OLT does not receive an uplink signal from an ONU (Optical Network Unit: a user side terminating apparatus) for a fixed time (Patent Literature 2).